Trouble Testing a Method that Uses a Singleton Dependency Pattern with Moq in .NET Core
I've tried everything I can think of but I'm having trouble with I'm integrating two systems and I'm optimizing some code but I'm working on a personal project and I'm currently working on unit tests for a service in my .NET Core application that relies on a singleton dependency..... The service calls a method from this singleton, and I'm having trouble mocking it correctly using Moq. The singleton is instantiated via a static property, and I want to ensure my tests are isolated. Here's what my code looks like: ```csharp public class MySingleton { private static MySingleton _instance; public static MySingleton Instance => _instance ??= new MySingleton(); public virtual string GetData() => "Real data"; } public class MyService { public string GetServiceData() { return MySingleton.Instance.GetData(); } } ``` In my test, I tried to mock the `GetData` method of the singleton like this: ```csharp [Test] public void GetServiceData_ShouldReturnMockedData() { var mock = new Mock<MySingleton>(); mock.Setup(m => m.GetData()).Returns("Mocked data"); // This line causes issues // Injecting the mock into the service somehow var service = new MyService(); var result = service.GetServiceData(); Assert.AreEqual("Mocked data", result); } ``` However, I keep getting an behavior stating that `want to create an instance of the abstract class or interface`. I’ve considered using a different design pattern, but since this singleton is heavily used throughout the application, refactoring it feels daunting. I've also attempted to use `Moq`'s `MockBehavior` to set up the mock, but to no avail. What’s the best way to handle testing in this scenario while maintaining the singleton design? Are there best practices to follow when dealing with singletons in unit tests? My development environment is Linux. Is there a better approach? Is there a better approach? This is my first time working with C# 3.11. Any suggestions would be helpful. I'm coming from a different tech stack and learning C#. Is there a better approach? I'm working on a mobile app that needs to handle this.